Well, I don't think most people take breaks and come back expecting to get faster more quickly but rather they just slow down the amount they cube for a little bit and go and do other stuff rather than consciously staying away from cubing.
I guess what I'm trying to say is that enforced breaks don't help that much (especially if you are already sub-15 or sub-10). It might also be a mental thing as people would expect to would've a lot slower than they were before but find they catch up to the times more quickly so perceive more progress than they would otherwise.
Lastly, it might also be that they start cubing more than they did previously or at least leading up to the break so they seem to get faster but they are doing more solves on order to. This is particularly likely if they stopped cubing for a little bit due to boredom or similar.